The present invention relates to position sensing devices. In particular, the present invention relates to a capacitive resolver for use as a position sensing device in satellite and terrestrial applications.
In satellite and terrestrial applications, it is often necessary to accurately position different instruments. For example, in satellite applications, accurate positioning of the satellite payload (e.g. telescopes, antennas, or other instruments) is essential. These instruments can be positioned by mounting them on a gimbal which includes a rotatable shaft. In order to accurately position these instruments, the rotational position of the shaft must be determined, and this can be accomplished by using a rotational position sensor.
Rotational position sensors provide position data to the motors which move the gimbal and the shaft. Typically, a rotational position sensor measures the position of the instrument, and this measurement is used to produce a motor commutation signal. The motors then use the motor commutation signal to determine the torque required to move the shaft in the desired direction.
Motor commutation is best accomplished using the output of two sinusoids in quadrature (i.e. two sinusoids separated in phase by 90 degrees). In position sensing applications, it is important to precisely determine position. However, this task is complicated with a sinusoidal output because there are two points on the sine curve that correspond to a given voltage. The addition of a cosine wave, which is 90 degrees out of phase from a sine wave, resolves this ambiguity and allows the position to be determined. For motor commutation, therefore, it is desirable to generate both a sine wave and a cosine wave and then to measure both of the corresponding voltages to unambiguously determine the rotational position of the shaft.
Capacitive sensors, which consist of parallel conducting plates, use changes in capacitance to measure changes in position. Capacitive sensors are often used as rotational position sensors due to their light weight and low cost. However, previous capacitive sensors were not capable of producing output of two sinusoids in quadrature, which is the preferred output for motor commutation.
Additionally, although some previously available rotational position sensors (such as inductive resolvers, wire-wound resolvers, and optical encoders) were capable of producing output of two sinusoids in quadrature, these devices were complex, heavy, and expensive. These types of rotational position sensors also tended to have difficulties with connecting wires becoming tangled around the rotating shaft, although the wire-wound resolver avoided this problem at the cost of added complexity. Finally, if a different motor speed was required for a particular application, it was difficult and expensive to mechanically retool these types of rotational position sensors.
A need has thus long existed in the industry for a lightweight, inexpensive capacitive resolver that allows the output of two sinusoids in quadrature, with flexibility in a number of output speeds.